The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with contrast-enhanced magnetic resonance angiography and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with bolus detection in other magnetic resonance applications.
Measurement of blood flow, in vivo, is important for the functional assessment of the circulatory system. Angiography has become a standard technique for making such functional assessments. Magnetic resonance angiography (MRA) provides detailed angiographic images of the body in a non-invasive manner. In conventional MRA, which does not use contrast agents, magnetic resonance signal from flowing blood is optimized, while signal from stationary blood or tissue is suppressed. In contrast-enhanced MRA, a T1-shortening contrast agent is injected into the blood stream in order to achieve contrast between flowing blood and stationary tissue. When data is collected using a short TR, short TE echo sequence, the blood appears bright, while the stationary tissue appears dark.
Current contrast-enhanced 3D MRA techniques produce excellent images of the arteries if the center of k-space is acquired during peak concentration of the contrast agent in the arteries. However, obtaining high quality images requires appropriate timing of the injection of the contrast agent relative to the start of image acquisition. If the center of k-space is acquired too early, maximum signal in the arteries will not be achieved. Conversely, if the center of k-space is acquired too late, the veins will be enhanced, causing the arteries to be obscured. Therefore, a premium is placed on reliable determination of bolus arrival.
In one prior art technique, a small trial volume of contrast agent is injected and a short two-dimensional acquisition is repeated at or near the region of interest to sense the arrival of the bolus of contrast agent. From this acquisition, the time of transit from the injection point to the region of interest is determined. A subsequent angiogram is started at the determined time after the injection of a full dose of the contrast agent. This technique is unreliable because the bolus travel time depends on variable patient factors, making the timing less than ideal. Inaccurate timing of bolus arrival produces a useless angiogram.
Another prior art technique sets up a short two-dimensional triggering acquisition upstream of the area of interest and a 3D time-of-flight acquisition at the area of interest. The contrast agent is injected and the triggering scan is repeated until the bolus is detected. After detection of the bolus, the 3D angiogram is started, with elliptical centric phase encoding from the center of k-space outward. This technique introduces the risk of false positives caused by patient motion. While false positives may be reduced by applying bimodal presats, the application of such bimodal presats increases sampling time and introduces additional latency between detection of the bolus and the angiogram acquisition.
Another prior art technique employs a 3D angiogram to generate a cine loop using view sharing, thus avoiding the need to detect the arrival of the bolus. However, this method requires a short total acquisition time for each frame in the cine loop in order to avoid unacceptable image artifacts and to ensure that the bolus is present for a significant portion of the acquisition. Ultimately, the short total acquisition time results in limited image resolution.
Therefore, a need exists for a contrast-enhanced magnetic resonance angiography method having zero latency between the detection of the bolus and the acquisition of a high-resolution magnetic resonance angiogram. The present invention contemplates a new and improved contrast-enhanced MRA method which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of magnetic resonance angiography (MRA) includes injecting a subject with a bolus of a magnetic resonance contrast agent upstream from a volume of interest. Magnetic resonance is excited in the volume of interest and magnetic resonance echoes, which generate magnetic resonance signals, are induced. A central portion of k-space is continuously sampled, creating a plurality of central data sets. The arrival of a leading edge of a bolus of contrast agent within the volume of interest is detected. After detecting the arrival of the leading edge of the bolus, portions of k-space peripheral to the central portion are sampled, creating peripheral data sets. A magnetic resonance angiograph is reconstructed from (i) the last central data set collected before the detection of the bolus and (ii) the peripheral data sets collected after the detection of the bolus.
In accordance with another aspect of the present invention, a method of acquiring a three-dimensional MRA within a volume of interest includes determining a trigger point within the volume of interest for a three-dimensional magnetic resonance data acquisition. An intravascular magnetic resonance contrast agent is administered to the patient""s circulatory system so as to enhance magnetic resonance imaging of blood vessels. Magnetic resonance data is acquired from the volume of interest and the arrival of the leading edge of a bolus of contrast agent at a trigger point is detected from the acquired magnetic resonance data. After detecting the arrival of the bolus at the trigger point, a series of three-dimensional magnetic resonance acquisitions are performed. The acquired magnetic resonance data is then reconstructed into a three-dimensional magnetic resonance angiograph.
In accordance with another aspect of the present invention, a method of detecting the arrival of a bolus of magnetic resonance contrast-enhancing agent in a volume of interest includes continuously sampling a centrally encoded portion of k-space, generating a plurality of central data sets. A maximum intensity projection is generated from a portion of each central data set. The generated maximum intensity projection is compared to a predetermined projection threshold value. In response to the comparing step, either the sampling of higher phase encode portions of k-space is triggered or the current central data set is discarded and the above steps are repeated.
In accordance with another aspect of the present invention, a magnetic resonance angiography system (MRA) for imaging a subject into which a magnetic resonance contrast-enhancing agent is injected includes a magnet for generating a temporally constant magnetic field through a volume of interest. A radio frequency transmitter excites and inverts magnetic dipoles in the volume of interest to generate a train of magnetic resonance echoes. Gradient magnetic field coils and a gradient magnetic field controller generate at least phase and read magnetic field gradient pulses in orthogonal directions across the volume of interest. A receiver receives and demodulates the magnetic resonance echoes to produce a series of k-space views. The system further includes a central memory portion for storing centrally-encoded k-space views and a peripheral data memory portion for storing higher phase encode views which are peripherally around the central phase encode views. A bolus detection processor detects an arrival of a leading edge of a bolus of the contrast-enhancing agent. A reconstruction processor reconstructs at least a portion of the centrally encoded k-space views and the higher phase encode views.
In accordance with a more limited aspect of the present invention, the bolus detection processor includes a maximum intensity projection processor for processing at least a portion of the centrally encoded k-space views into a maximum intensity projection of a portion of the volume of interest through which the bolus enters the volume of interest onto a line. Further, an edge detector detects the arrival of the contrast-enhancing agent bolus from the projection.
One advantage of the present invention resides in a zero latency between detection of the bolus and the acquisition of the angiogram.
Another advantage of the present invention is that it improves the reliability of bolus arrival determinations.
Another advantage of the present invention resides in the reduction of patient-movement artifacts.
Another advantage of the present invention resides in more precise control of central phase encode view acquisition.
Still another advantage of the present invention is that it interleaves triggering views with high resolution views.
Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.